Damascus has been the target of a number of bombs in past months amid continuing anti-government unrest. The last deadly blast there was at the end of April, when 11 people were killed in a suicide bomb attack near a mosque.

There has been intense shelling by the Syrian army overnight in the city of Homs further north.

Thick black smoke could be seen at a distance on the east side of the capital, the Associated Press news agency reports.

The shelling in Homs has been described by opposition activists as the worst in weeks.

Eleven United Nations monitors are now stationed in Homs to try to implement a ceasefire there.

But a BBC correspondent in Homs says they have an impossible task, with both sides breaking the truce.